Solid state imagers, including charge coupled devices (CCD) and CMOS imagers, have been used in photo imaging applications. A solid state imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, which may be a photogate, photoconductor or a photodiode having a doped region for accumulating photo-generated charge.
During the manufacture of solid state imagers, the creation of defective pixels is unavoidable. These defective pixels, if not corrected, can cause severe degradation of image quality and, as a result, decrease the yield of parts during production. Thus, minimization of these defects during fabrication will yield a higher quality product. However, it is usually less expensive to make a device (e.g., semiconductor imager device) using less precise manufacturing tolerances. Devices that are produced using less precise manufacturing tolerances, on the other hand, have a higher probability of defects. Typical semiconductor fabrication rules define some tradeoff between the quality (i.e., lack of defects) and cost of manufacture. The manufactured semiconductor devices are tested for defects, and any semiconductor device having more than a certain number of defects is usually discarded.
Image acquisition semiconductor devices are sensitive to defects and a sensor with defects may not yield aesthetically pleasing images. It is especially evident when defects are located in low frequency areas or at edges. An edge in images are areas with strong intensity contrasts. A bad pixel in an image acquisition semiconductor device will show up as a bad area on the acquired image. The defective pixels may not work at all or, alternatively, they may be significantly brighter or dimmer than expected for a given light intensity. Depending on the desired quality and the intended application, a single defective pixel may sometimes be sufficient to cause the device containing the pixel to be discarded.
In most instances, however, a small percentage of defective pixels can be tolerated and compensated for. Numerous techniques exist for locating and correcting single defective pixels in an image acquisition semiconductor device.
One simple technique for correcting single defective pixels involves taking a signal from each pixel in an array and storing the pixel signal values in memory. During image processing, the saved value for a defective pixel can be replaced by the calculated signal value of the neighboring pixels of the defective pixel. These simple methods, however, are not viable for all pixel defects, such as, for example, those suffering from excessive dark current. Other more complicated methods have been devised to correct single defective pixels, including dark current pixels. For example, see the method discussed in the paper submitted by B. Dierickx and G. Meyanants “Missing Correction Method for Image Sensors,” submitted for Europto-SPIE/AFPAEC May 18-21, 1998.
Correction of multiple defective pixels in a small area of an array, termed “cluster defects” or “defective pixel clusters,” however, still remains a significant challenge. Accordingly, there is a need and desire for a method capable of correcting defective pixel clusters to improve the yield of imager manufacturing.